The telecommunication standard IS-95 (PN-3384), "Personal Station - Base Station Compatibility Requirements for 1.8 to 2.0 GHz Code Division Multiple Access (CDMA) Personal Communications Systems",TIA & ATIS 1994, which is known to tie person skilled in the art, describes the most important technical characteristics of a cellular radio telecommunication system, in which CDMA (Code Division Multiple Access) serves as the multiple access method that divides the radio capacity of the system among several users. The standard has been accepted as the basis of commercial cellular networks in the USA It does not restrict the size of cells of a cellular network, so it allows the operation of overlayed radio communication systems, where the first system is an extensive cellular network with very wide coverage, relatively large cells and, consecutively, high transmission power at both base stations and mobile stations. The second one of the overlayed systems in the sense of the present patent application is an indoor cellular network with very small cell size and very low average transmission power. The second system does not have to be literally indoors, but the concept "indoor cellular network" or "indoor system" is widely accepted in the field to describe this particular kind of a system, which usually operates in office environments.
Indoor systems typically operate either in a different frequency band than outdoor systems or in an overlayed manner, where same frequency bands are used in both systems. The former approach requires frequency planning, in which some central authority allocates the available frequency bands to competing user networks. Frequencies that are exclusively allocated to a certain system but remain unused due to low traffic in that system represent a waste of common radio resources. The latter option, in which the indoor and outdoor systems share common frequency bands, is therefore regarded as superior, because dynamic channel allocation inside said frequency bands between active users uses the available capacity more effectively. There remains the problem of intersystem interference, which means that transmissions in one system appear at the receivers of the other system causing errors and signal corruption. The biggest concern is the interference due to the Outdoor Mobile Stations (OMS's) and Outdoor Base Stations (OBS's) to the indoor system, because their transmission power is much higher than that of the Indoor Base Stations (IBS's) and Indoor Mobile Stations (IMS's) However, it is also beneficial to keep the average transmission power in the indoor system as low as possible, in order to save power in battery-operated mobile stations and to reduce the possibility of indoor to outdoor interference, as well as intrasystem interference between the different stations of the indoor system.
There appears to be no feasible prior art solution that would allow the effective sharing of same frequency bands between overlayed indoor and outdoor radio communication systems while simultaneously preventing mutual interference between the two systems, if both systems rely at least partly on the IS-95 standard in their air interface.